Mr Crowley has not attended the parliament at all since his re-election in 2014, owing to ill-health, and he remains in hospital. Under European Parliament rules, Mr Hartley would take his seat if Mr Crowley resigned.

In an open letter to the Fianna Fáil leader, seen by The Irish Times, Mr Hartley is critical of both Mr Crowley’s lengthy absence from the parliament, and of Mr Martin’s refusal to intervene.

Mr Crowley stood as a Fianna Fáil MEP but refused to comply when the party switched from the European Conservatives and Reformists (ECR) group in the European Parliament to the alliance of liberals. He remains a member of the ECR group, and as such is outside the Fianna Fáil parliamentary party, the party says.

Mr Hartley points out in his letter that this means that in his absence, his speaking time is taken up by members of the British Conservative Party and the DUP, who are members of the ECR group – something he say is “directly against the interests and political will of the Irish people and the Irish State”.

“We timidly and freely give our [European Parliament] vote and our voice to the Unionist & Tory opposition, an opposition that is advocating for Brexit,” he writes in his letter.

“Everyone wishes Mr Crowley well, however, to say that you haven’t got involved or wish to get involved in this situation does not serve the cause of representative democracy,” Mr Hartley wrote to Mr Martin. “It is flippant to Ireland, its citizens and the party. Ireland at this time of Brexit deserves a more honest and engaging approach. Brexit and politics pauses for no-one.

“Mr Crowley’s prolonged and ongoing 6 year+ absence is symptomatic of much that is publicly perceived to be wrong with Irish politics. A consequence of which is a resulting spread of Irish public EU antipathy, which, after all, helped contribute to Brexit becoming a reality in the UK. We need to be honest and to be seen to be honest with our electorate. This I believe is damaging the very fabric of the EP and is leading to scepticism amongst young voters and in turn will damage European institutions,” Mr Hartley wrote.

Working from hospital

Mr Crowley has been absent due to ill-health since the European Parliament elections in 2014, and for long periods of the previous parliamentary term between 2009 and 2014, say colleagues. He has been confined to a wheelchair since an accident in his teens.

Mr Crowley’s office did not respond to a request for comment, but has previously said that he continues to work from hospital. He will make a statement once he emerges from hospital, his office has said, though it is understood he has been in hospital for many months.

A Fianna Fáil spokesman said: “Following Brian Crowley’s decision to join the ECR grouping in the European Parliament, he was no longer a member of the Fianna Fáil Parliamentary Party.

“However, he remains a Fianna Fáil party member. Brian was elected to the European Parliament in 2014 with the single largest vote in Ireland, and while he is currently suffering from ill health, we understand that his office continues to provide a full range of services to constituents.